The present invention relates to thermal transfer of organic materials from a donor element to a receiving device, such as an OLED device.
In color or full-color organic electroluminescent (EL) displays (also known as organic light-emitting diode devices, or OLED devices) having an array of colored pixels such as red, green, and blue color pixels (commonly referred to as RGB pixels), precision patterning of the color-producing organic EL media are required to produce the RGB pixels. The basic OLED device has in common an anode, a cathode, and an organic EL medium sandwiched between the anode and the cathode. The organic EL medium can consist of one or more layers of organic thin films, where one of the layers is primarily responsible for light generation or electroluminescence. This particular layer is generally referred to as the emissive layer of the organic EL medium. Other organic layers present in the organic EL medium can provide electronic transport functions primarily and are referred to as either the hole transport layer (for hole transport) or electron transport layer (for electron transport). In forming the RGB pixels in a full-color OLED display panel, it is necessary to devise a method to precisely pattern the emissive layer of the organic EL medium or the entire organic EL medium.
A suitable method for patterning high-resolution OLED displays has been disclosed in commonly-assigned U.S. Pat. No. 5,851,709 by Grande et al. this method is comprised of the following sequences of steps: 1) providing a substrate having opposing first and second surfaces; 2) forming a light-transmissive heat-insulating layer over the first surface of the substrate; 3) forming a light-absorbing layer over the heat-insulating layer; 4) providing the substrate with an array of openings extending from the second surface to the heat-insulating layer; 5) providing a transferable color-forming organic donor layer formed on the light-absorbing layer, 6) precision aligning the donor substrate with the display substrate in an oriented relationship between the openings in the substrate and the corresponding color pixels on the device; and 7) employing a source of radiation for producing sufficient heat at the light-absorbing layer over the openings to cause the transfer of the organic layer on the donor substrate to the display substrate. A problem with the Grande et al. approach is that patterning of an array of openings on the donor substrate is required. Another problem is that the requirement for precision mechanical alignment between the donor substrate and the display substrate. A further problem is that the donor pattern is fixed and cannot be changed readily.
Littman and Tang (commonly-assigned U.S. Pat. No. 5,688,551) teach the patternwise transfer of organic EL material from an unpatterned donor sheet to an EL substrate. A series of patents by Wolk et al (U.S. Pat. Nos. 6,114,088; 6,140,009; 6,214,520; and 6,221,553) teaches a method that can transfer the luminescent layer of an EL device from a donor element to a substrate by heating selected portions of the donor with a laser beam. Each layer is an operational or non-operational layer that is utilized in the function of the device.
In these processes a donor containing the electroluminescent materials is heated by radiation and transferred to a receiver which may already contain a portion of the active device. The device may then be finished by the application of further layers. This process allows the patterning of colors by the use of a suitable donor which contains an electron or hole conductors host and a dopant. The final light emitting device must have the dopant mixed together to give a good emission. It is difficult to co-evaporate two or more materials simultaneously and maintain a constant controlled ratio. The resulting emission from these radiation-transferred devices also have need for improved efficiency.
Deboer and Spahn (commonly-assigned U.S. Pat. No. 5,244,770) teaches a donor element for color transfer in the field of color printing. They introduce the concept of an anti-reflection layer located between a transparent support, and a heat absorbing metal layer. This element is used to transfer a dye layer comprising a binder and a sublimable dye. The use of a binder is common in the field of color printing, but is inappropriate in the fabrication of OLED devices. It is difficult to transfer an organic material without contamination from the binder. In a color print, low levels of contamination can be tolerated, but such contamination would be unacceptable in an electroluminescent device such as an OLED, compromising lifetime, efficiency, and appearance. Furthermore, the variety of systems covered by commonly assigned U.S. Pat. No. 5,244,770 are not uniformly appropriate for patterning a device such as OLED. As a manufacturing process, throughput is critical, and only the most absorptive donors are attractive, as they will maximize throughput in the manufacturing process. Finally, the preferred sublimation process of materials for a device such as OLED is frequently near the threshold power required for the transfer of material, due to the fact that excessive power can lead to contamination of the device, or degradation of the transferred material. In order to operate near the threshold of transfer, it is a requirement that the donor efficiency, and therefore the donor absorption be uniform over the area of the donor. This requirement is often frustrated by the interference of the laser light reflected off of the absorbing layer of the donor with the laser light reflected off of the air-support interface.
It is therefore an object of the present invention to provide a very high absorption laser thermal substrate with low micro absorption variation onto which dyes or other organic materials can be coated.
This object is achieved in a method of making a high absorption donor substrate which can be coated with one or more organic material layers and for use in providing one or more OLED materials to an OLED device in response to laser light substantially within a predetermined wavelength range, includes:
(a) providing a transparent support element;
(b) providing an absorber anti-reflection layer over the transparent support element, the anti-reflection layer selected to have the real portion of its index of refraction greater than 3.0, and a thickness selected to be near the first reflectivity minimum at the wavelength of interest;
(c) providing a metallic heat-absorbing layer over the anti-reflection layer for absorbing laser light which passes through the transparent support element and the anti-reflection layer;
(d) selecting the transparent support element, the anti-reflection layer, and the metallic heat-absorbing layer to have an average reflectivity of less than 10%, and the micro reflectivity variation due to variations in the thickness of the transparent support element of less than 10% at the wavelength of interest; and
(e) providing one or more organic material layers in the absence of a binder material, over the metallic heat-absorbing layer which include organic material(s) which are transferable to an OLED device.
A tuned high absorption donor substrate for laser-thermal-transfer can have a high efficiency absorber which can absorb 95% to 100% of the light striking it. This improves optical efficiency of a donor element by a factor of two over simple donor substrates for laser-thermal-transfer with a simple metallic heat-absorbing layer as the absorber, and thus provides greater transfer efficiency at a given light intensity. The very low reflectivity absorber greatly reduces the variability in the absorption efficiency of the donor substrate, and allows a uniform transfer of organic material from a donor element made from the donor substrate even when operating near the transfer threshold. OLED materials are known to be sensitive to excessive heating, so the best transfer conditions are expected to be near the transfer threshold for a wide variety of systems. A donor with an average reflectivity of greater than only 10% can have a variability in the transfer threshold of up to 18%. Furthermore, since contamination is critical in electroluminescent devices such as an OLED, it is advantageous to perform transfers for OLED devices with an organic layer which is transferred in its entirety, as opposed to an organic layer with an inert binder which is either partially transferred or not transferred at all.